Our long-term objective is to increase our understanding of the regulation of nephron function by exploiting the unique renal structure of birds, the highly specialized renal structure of certain desert rodents, and the specialized transport properties of the nephrons of reptiles. The avian kidney has a population of nephrons resembling reptilian nephrons that function independently of each other, do not contribute directly to the concentrating mechanism, and can cease filtering altogether under some circumstances and a population of nephrons resembling mammalian nephrons that function together in the concentrating mechanism, do not normally cease filtering, but may alter their filtration rates. The kidneys of certain tiny desert rodents have juxtamedullary nephrons that are several times larger than the superficial cortical nephrons and a very long papilla and represent an intermediate state between the avian kidney and the more commonly studied mammalian kidneys. In contrast to mammalian nephrons, reptilian and avian nephrons secrete phosphate under the control of parathyroid hormone. Our primary goals under this broad objective involve studies of: 1) the regulation of nephron function, including the regulation of single nephron filtration rates (SNGFR) and tubular transport, in avian, desert rodent, and reptilian kidneys; 2) the relationship of single nephron function to the concentrating and dilut ng mechanism; and 3) the patterns and regulation of renal portal flow within the avian kidney. We are performing and plan to continue micropuncture and microperfusion (both in vivo and in vitro) studies of specific factors regulating SNGRFs (e.g., sodium, chloride, antidiuretic hormone, and distal tubule-glomerular feedback) and of factors influencing tubular transport, including phosphate transport (e.g., parathyroid hormone). We also plan micropuncture studies of loop of Henle function and the effects of alterations in SNGFR on this function. Electron microprobe analysis is being used to permit measurement of multiple inorganic ions in the tiny samples obtained. We also plan in vivo studies of avian renal portal blood flow patterns by silicone injection techniques and micropressure measurements and in vitro studies of specific factors (e.g., antidiuretic hormone) regulating the renal portal valve.